In general, rubber compositions for tire tread, in particular, for high performance tire tread, should have good balance of the properties such as high wear resistance, good wet skid resistance, good dry skid properties, good high speed stability, and good durability at high speed, without compromising steering.
At the same time, the needs of consumers have become diversified. Accordingly, numerous types of high performance tires are made on a small scale, and it is necessary to provide rubber compositions which emphasize one or more of the above-discussed properties. For example, compounds having remarkable driving grip properties, but not the best wet skid resistance have been developed, as have been those emphasizing wear resistance or wet skid properties. Recently, polymers or rubber compositions having good gripping properties, good wear resistance and good durability at high speed are desired for use as high performance tires.
To provide these rubber compositions, the relationship between wet skid resistance or dry grip properties and viscoelasticity of rubber compositions has been studied. It has been found that in order to improve wet skid resistance, tan.delta. at 10 to 20 Hz (in a low temperature region, i.e., around 0.degree. C.), in particular, should be increased. Also, to improve driving grip properties, tan.delta. around 50 to 70.degree. C. should be increased in order to increase hysteresis loss.
It has also been found that in order to improve wear resistance and durability at high speed, tensile strength and elongation at break (both at high temperature, i.e., around 100.degree. C.) in particular, should be improved. The improvement in these breaking properties leads to improvement not only in wear resistance and durability at high speed but also, by controlled compounding, improvement in wet skid resistance and in dry grip properties. Most other properties can also be controlled by improving the breaking properties.
By improving certain properties of the polymer or polymer composition used for tire tread, such as the mechanical durability discussed above, performance of a tire can be improved. In this regard, it is known to improve these properties using an existing rubbery polymer or a novel polymer, i.e., by adding such polymers to the tire tread composition to make a novel composition.
As examples of the former method, a butyl rubber has been added to a butadiene-styrene copolymer, (see JP-A-62- 143945), as has been a polynorbornene (see JP-A-62-143945 or see JP-A-2-142838), a polyisoprene (see JP-A-63-132949), or a cumarone-indene resin (JP-A-62-1735). As examples of the latter method, a butadiene-styrene rubber has been modified with a diphenylmethyl alcohol derivative (see JP-A-60-61314), a diblock butadiene-styrene copolymer has been used as a rubbery component (see JP-A-1-131258), a butadiene-styrene copolymer has been polymerized using an organolithium compound and certain organic compounds such as potassium butoxide, as a rubbery component (see JP-B-44-20463), and a butadiene-styrene copolymer has been polymerized (using a potassium salt, as a randomizer) as rubbery component (see JP-A-3-239737).
A butadiene-styrene copolymer having less than 40% styrene mono-segments and at the most 10% of styrene chemical block, having 8 or more styrene units based on total bound styrene content with low vinyl linkage in the butadiene portion, is known (see JP-A-3-239737). According to this document, however, if tetrahydrofuran is used to increase the vinyl content, styrene blocks will be broken and the styrene mono-segment content increases.
However, these methods do not result in a rubber composition having satisfactory performance as a tire.